Moorland grouse - Flubendazole use for parasitic worm control: preliminary environmental assessment
A report assessing the potential impact on the wider environment of the use of flubendazole in medicated grit.
Appendix B – Equilibrium partitioning for sediment & soil toxicity assessment
The approach to equilibrium partitioning (EqP) is taken from the European Commission Common Implementation Strategy 27 on deriving EQS (EC, 2018) and underlying REACH guidance (EC, 2008).
1) Sediment
As there are not enough data to calculate a water PNEC, the lowest available pelagic chronic endpoint has been used instead as an indicator of sediment toxicity. This assessment assumes that sediment-dwelling organisms are of similar sensitivity to pelagic organisms, that sediment pore water concentrations are related to water column concentrations (ie partitioning processes are at equilibrium), and that exposure occurs mainly via the pore water (for substances with log Kow <5).
The partition coefficient between solid-water in sediment is calculated by:
𝐾𝑝𝑠𝑒𝑑 = 𝐹𝑜𝑐𝑠𝑒𝑑 × 𝐾𝑜𝑐
Where 𝐾𝑝𝑠𝑒𝑑 is the partition coefficient between solid-water in sediment (m3 m-3), 𝐹𝑜𝑐𝑠𝑒𝑑 is the weight fraction of organic carbon in sediment (kg kg-1) set as default 0.05 and 𝐾𝑝𝑠𝑒𝑑 is the partition coefficient between organic carbon and water (L kg-1), where for flubendazole a value of 10,000 (log Koc 4) L kg-1 has been used. The 𝐾𝑝𝑠𝑒𝑑 for flubendazole is therefore:
0.05 × 10000 = 500
This value is used to derive the partition coefficient between sediment and water as:
𝐾𝑠𝑒𝑑−𝑤𝑎𝑡𝑒𝑟 = 𝐹𝑎𝑖𝑟𝑠𝑒𝑑 × 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 + 𝐹𝑤𝑎𝑡𝑒𝑟𝑠𝑒𝑑 + 𝐹𝑠𝑜𝑙𝑖𝑑𝑠𝑒𝑑 × 𝐾𝑝𝑠𝑒𝑑 ÷ 1000 × 𝑅𝐻𝑂𝑠𝑜𝑙𝑖𝑑
Where, 𝐾𝑠𝑒𝑑−𝑤𝑎𝑡𝑒𝑟 is the partition coefficient between sediment and water, 𝐹𝑎𝑖𝑟𝑠𝑒𝑑 is the fraction of air in sediment (m3 m-3) set at default 0, 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 is the air-water partition coefficient (m3 m-3), 𝐹𝑤𝑎𝑡𝑒𝑟𝑠𝑒𝑑 is the fraction of water in sediment (m3 m-3) set at default 0.8, 𝐹𝑠𝑜𝑙𝑖𝑑𝑠𝑒𝑑 is the fraction of solids in sediment (m3 m-3) set at 0.2 and 𝑅𝐻𝑂𝑠𝑜𝑙𝑖𝑑 is the density of the solid phase (kgsolid msolid -3) set at default of 2500. 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 is calculated using the equation below:
𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 = 𝐻 ÷ 𝑅 × 𝑇𝐸𝑀𝑃
Where, H is the Henry’s law constant (Pa m3 mol-1) which is 2.73 x 10-13 (based on EPIsuite prediction) for flubendazole, R is the gas constant (Pa m3 mol-1 K-1) set at default 8.314 and TEMP is the environmental temperature (K) set at 285. 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 for flubendazole is therefore:
2.73 × 10−13 ÷ 8.314 × 285 = 8.98 × 10−17
The 𝐾𝑠𝑒𝑑−𝑤𝑎𝑡𝑒𝑟 can therefore be calculated as:
0 × 8.98 × 10−17 + 0.8 + 0.2 × 500 ÷ 1000 × 2500 = 250.8 m3 m-3
The sediment water coefficient (𝐾𝑠𝑒𝑑−𝑤𝑎𝑡𝑒𝑟) can then be used to determine the wet weight equilibrium partitioning quality standard for sediment (𝑄𝑆𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡,𝐸𝑞𝑃,𝑤𝑤) as:
𝑄𝑆𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡,𝐸𝑞𝑃,𝑤𝑤 = 𝐾𝑠𝑒𝑑−𝑤𝑎𝑡𝑒𝑟 ÷ 𝑅𝐻𝑂𝑠𝑒𝑑 × 𝑄𝑆𝑓𝑤,𝑒𝑐𝑜 × 1000
Where, 𝑄𝑆𝑠𝑒𝑑𝑖𝑚𝑒𝑛𝑡,𝐸𝑞𝑃,𝑤𝑤 is the wet weight equilibrium partitioning quality standard for sediment, 𝑅𝐻𝑂𝑠𝑒𝑑 is the bulk density of wet sediment which is set as default 1300 kgww m-3 and 𝑄𝑆𝑓𝑤,𝑒𝑐𝑜 is the freshwater quality standard. In this case there is no quality standard or PNEC derived for water, so the lowest chronic endpoint (0.0025 mg L-1) has been used instead to give an indication of possible toxicity to sediment dwelling organisms calculated in the same way:
250.8 ÷ 1300 × 0.0025 × 1000 = 0.482 𝑚𝑔.𝑘𝑔 −1
2) Soil
The same process can be used to estimate toxicity to soil dwelling organisms following guidance developed for the REACH Regulation (EC, 2008). The same principles and caveats apply, although less validation work has been undertaken for the application of EqP for the soil compartment and there are indications that the approach is more uncertain for the soil compartment than sediment.
The partition coefficient between solids and water in soil is calculated by:
𝐾𝑝𝑠𝑜𝑖𝑙 = 𝐹𝑜𝑐𝑠𝑜𝑖𝑙 × 𝐾𝑜𝑐
Where 𝐾𝑝𝑠𝑜𝑖𝑙 is the partition coefficient between solids and water in soil (m3 m-3), 𝐹𝑜𝑐𝑠𝑜𝑖𝑙 is the weight fraction of organic carbon in soil (kg kg-1) set as default 0.02 and 𝐾𝑜𝑐 is the partition coefficient between organic carbon and water (L kg-1), where for flubendazole a value of 10,000 (log Koc 4) L kg-1 has been used. The 𝐾𝑝𝑠𝑜𝑖𝑙 for flubendazole is therefore:
0.02 × 10000 = 200
This value is used to derive the partition coefficient between soil and water as:
𝐾𝑠𝑜𝑖𝑙−𝑤𝑎𝑡𝑒𝑟 = 𝐹𝑎𝑖𝑟𝑠𝑜𝑖𝑙 × 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 + 𝐹𝑤𝑎𝑡𝑒𝑟𝑠𝑜𝑖𝑙 + 𝐹𝑠𝑜𝑙𝑖𝑑𝑠𝑜𝑖𝑙 × 𝐾𝑝𝑠𝑜𝑖𝑙 ÷ 1000 × 𝑅𝐻𝑂𝑠𝑜𝑙𝑖𝑑
Where, 𝐾𝑠𝑜𝑖𝑙−𝑤𝑎𝑡𝑒𝑟 is the partition coefficient between soil and water, 𝐹𝑎𝑖𝑟𝑠𝑜𝑖𝑙 is the fraction of air in soil (m3 m-3) set at default 0.2, 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 is the air-water partition coefficient (m3 m-3), 𝐹𝑤𝑎𝑡𝑒𝑟𝑠𝑜𝑖𝑙 is the fraction of water in soil (m3 m-3) set at default 0.2, 𝐹𝑠𝑜𝑙𝑖𝑑𝑠𝑜𝑖𝑙 is the fraction of solids in soil (m3 m-3) set at 0.6 and 𝑅𝐻𝑂𝑠𝑜𝑙𝑖𝑑 is the density of the solid phase (kgsolid msolid -3) set at default of 2500. 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 is calculated using the equation below:
𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 = 𝐻 ÷ 𝑅 × 𝑇𝐸𝑀𝑃
Where, H is the Henry’s law constant (Pa m3 mol-1) which is 2.73 x 10-13 (based on EPIsuite prediction) for flubendazole, R is the gas constant (Pa m3 mol-1 K-1) set at default 8.314 and TEMP is the environmental temperature (K) set at 285. 𝐾𝑎𝑖𝑟−𝑤𝑎𝑡𝑒𝑟 for flubendazole is therefore:
2.73 × 10−13 ÷ 8.314 × 285 = 8.98 × 10−17
The 𝐾𝑠𝑜𝑖𝑙−𝑤𝑎𝑡𝑒𝑟 can therefore be calculated as:
0.2 × 8.98 × 10−17 + 0.2 + 0.6 × 200 ÷ 1000 × 2500 = 300.2 m3 m-3
The soil water coefficient (𝐾𝑠𝑜𝑖𝑙−𝑤𝑎𝑡𝑒𝑟) can then be used to determine the wet weight equilibrium partitioning quality standard for soil (𝑄𝑆soil,𝐸𝑞𝑃,𝑤𝑤) as:
𝑄𝑆soil,𝐸𝑞𝑃,𝑤𝑤 = 𝐾𝑠𝑜𝑖𝑙−𝑤𝑎𝑡𝑒𝑟 ÷ 𝑅𝐻𝑂𝑠𝑜𝑖𝑙 × 𝑄𝑆𝑓𝑤,𝑒𝑐𝑜 × 1000
Where, 𝑄𝑆soil,𝐸𝑞𝑃,𝑤𝑤 is the wet weight equilibrium partitioning quality standard for soil, 𝑅𝐻𝑂𝑠𝑜𝑖𝑙 is the bulk density of wet soil which is set as default 1150 kgww m-3 and 𝑄𝑆𝑓𝑤,𝑒𝑐𝑜 is the freshwater quality standard. In this case there is no quality standard or PNEC derived for water, so the lowest chronic endpoint (0.0025 mg L-1) has been used instead to give an indication of possible toxicity to soil dwelling organisms, but calculated in the same way:
300.2 ÷ 1150 × 0.0025 × 1000 = 0.653 𝑚𝑔.𝑘𝑔 −1
Contact
Email: john.gray@gov.scot
There is a problem
Thanks for your feedback